1. Field of the Invention
This invention relates to a semiconductor device having a feature in the electrode of laminated structure and having a preferable impurity-diffusion preventing function.
2. Description of the Related Art
Recently, polysilicon is widely used as a material of electrodes and wirings of semiconductor devices. However, with an increase in the integration density and operation speed of the semiconductor device, delay in the signal transmission due to the resistances of the electrode and wiring becomes an important problem.
This type of delay can be suppressed by lowering the resistances of the electrode and wiring. For example, in the case of a gate electrode of a MOS transistor, the delay can be suppressed by use of a polycide gate with two-layered structure of a metal silicide film and a polysilicon film.
However, in the generation of the gate length 0.25 .mu.m and in the succeeding generations, it is required to use a gate electrode having lower resistance than the polycide gate, and recently, a polymetal gate with laminated structure of a refractory metal film, reaction barrier layer and polysilicon layer has received much attention.
If tungsten (W) is used as refractory metal, the RC delay time can be significantly reduced since the specific resistance of tungsten is smaller than that of tungsten silicide (WSi.sub.x) by approximately one figure. Tungsten is a material which easily reacts with polysilicon in the heat treatment of approximately 600.degree. C., but no problem occurs since a reaction barrier layer is disposed between the W film and the polysilicon film.
Further, a metal gate of single-layered refractory metal film is expected to be widely used in the future rather than the polymetal gate. It is necessary to use refractory metal in order to lower the resistance of the gate electrode.
However, the refractory metal such as tungsten tends to be easily oxidized and, for example, tungsten is oxidized at approximately 400.degree. C. The oxide of tungsten is an insulator and tungsten causes the cubical expansion with oxidation.
Generally, in the LSI manufacturing process, it is necessary to use a step of effecting re-oxidation for enhancing the reliability of an oxide film such as a gate oxide film after the gate electrode pattern is formed. For example, in the case of a polysilicon gate, a polysilicon film is formed on a silicon substrate and patterned into a gate electrode, and then an oxide portion called a bird's beak is formed on the end portion of the gate oxide film. As a result, since the lower end portion of the gate electrode is rounded and the electric field around the gate portion can be reduced, the characteristic and reliability of the element can be enhanced. The re-oxidation step is hereinafter referred to as post-oxidation.
If this type of post-oxidation is applied to a polycide gate using WSi.sub.x as metal silicide, WSi.sub.x which contains rich Si in comparison with the normal composition x=2.0 is used as WSi.sub.x, and therefore, excessive silicon in WSi.sub.x is oxidized in the post-oxidation step to form SiO.sub.2 on the WSi.sub.x surface and the same insulating effect as that obtained by the oxidation method for crystal silicon can be attained.
If this type of post-oxidation is applied to a polymetal gate using W as refractory metal, W is oxidized in the normal oxidation step and WO.sub.3 is formed in the normal oxidation step. At this time, since large cubical expansion occurs, the film may be removed and the succeeding step cannot be continuously effected.
Further, oxidation of W is started by an oxidant such as O.sub.2 and H.sub.2 O introduced from the atmosphere before the oxidation step is started and the same problem may occur. Therefore, in the case of polymetal gate, it is necessary to use the technique (selective oxidation technique) for oxidizing only silicon without oxidizing refractory metal in the post-oxidation step.
A selective oxidation method for selectively oxidizing only silicon without oxidizing the exposed portion of refractory metal in a case where the exposed portion of silicon and the exposed portion of refractory metal such as W are both present on the same substrate as in the case of polymetal gate is known (Jpn. Pat. Appln. KOKAI Publication No. 60-9166).
The selective oxidation method is a method for effecting the oxidation with the partial pressure ratio of H.sub.2 O/H.sub.2 kept constant when the oxidation is effected in the mixed atmosphere of H.sub.2 O which is an oxidant and H.sub.2 which is a reducer.
As an example of application of the above technique, there is a report (R. F. Kwasnick et al., J. Electrochem. Soc., Vol 135, pp 176 (1988)) that the metal gate of a single layer of W is oxidized in the H.sub.2 /H.sub.2 O atmosphere. According to the result of experiments made by the reporters, when a sample obtained by stacking a W film (gate electrode) of 200 nm thickness on a thin silicon oxide film (gate oxide film) of 5 nm thickness was used and an oxidation process was effected at 900.degree. C. for approximately 30 minutes in the H.sub.2 /H.sub.2 O atmosphere, the thickness of the silicon oxide film lying directly under the W film was increased to 20 nm.
The phenomenon is caused by diffusion of the oxidant through the grain boundary of the W film. That is, the above selective oxidation technique does not oxide the W film, but silicon in the silicon oxide film lying directly under the W film is oxidized. Therefore, when the above selective oxidation technique is applied to the metal gate, the film thickness of the gate oxide film is increased, thereby causing a serious problem that the driving ability of a transistor is lowered.
When considering that the above selective oxidation technique is applied to a polymetal gate with laminated structure of a W film and a polysilicon film, it can be easily supposed that the polysilicon film lying directly under the W film will also be oxidized. Oxidation of the polysilicon film on the interface between the W film and the polysilicon film causes an increase in the contact resistance in the interface, causing a problem that the RC delay is increased.
However, the selective oxidation using the mixed gas of H.sub.2 O gas and H.sub.2 gas has the following problem.
The H.sub.2 gas which is a reducer explodes at temperatures of 600.degree. C. or more in the density range of 4% to 75%. On the other hand, oxidation of silicon is generally effected at a high temperature of 600.degree. C. or more. Therefore, the selective oxidation method using the mixed gas of H.sub.2 O gas and H.sub.2 gas has a problem in its safety.
For example, a temperature rise occurring when hydrogen of 60% is completely burnt is 3500.degree. C., and at this time, the volume expands by 4.3 times, and as a result, not only the device for oxidation is broken but also the surrounding is put into a dangerous condition.
For this reason, when the selective oxidation method is used, it is necessary to provide a device having a safety mechanism. As the safety mechanism, a mechanism for maintaining the reaction chamber in such a state that the reaction chamber will not explode or a mechanism capable of maintaining the safe state even if oxygen is introduced for some reason is necessary. Therefore, in order to actually use the above selective oxidation method, the above safety mechanism is required and the whole device is made complicated and the cost thereof becomes high.
As described above, in order to lower the resistance of the gate electrode, the electrode structure having a high electrical conductivity and a high degree of matching with the gate insulating film and the substrate may be formed by laminating metal with high electrical conductivity on polysilicon, but the gate electrode obtained by a combination with normal metal cannot withstand high temperatures set in the LSI manufacturing process. Particularly, in the self-aligned ion-implantation technique using the gate electrode as a mask and introduced together with miniaturization of elements and enhancement of the operation speed, the activation heat treatment after ion-implantation must be effected after formation of the gate electrode, and therefore, the gate electrode is required to have high resistance to heat.
Furthermore, in the high temperature processes at 800 to 900.degree. C. after the ion-implantation, including the above-mentioned oxidation, owing to the thermal diffusion of Si atoms or doped impurity atoms from the polysilicon to the refractory metal or a silicide thereof, the problem arises that a gate depletion is induced due to impurity concentration lowering in the silicon layer, or a work function in a CMOSFET (complemental MOSFET) is varied by the fact that impurities in n and p regions are mutually diffused through the above refractory metal or a silicide thereof, thereby varying a threshold voltage of the CMOSFET.